Capacitor power supply units form power supply units of simple construction for directly deriving the operating voltage needed for electronics from the mains voltage without the need for transformers or DC isolation. A series circuit comprising a mains capacitor, a series resistor and a rectifier diode is directly connected to the mains voltage, in which case a desired operating voltage which is used to operate a controller and/or a small electrical load is tapped off via a zener diode and preferably a smoothing capacitor, for example. In this case, the operating voltage derived from the mains voltage should be adapted to the permissible operating voltage of the small load.
One disadvantage of such capacitor power supply units is that the maximum output current of the power supply unit is predefined by the specific components of the capacitor power supply unit. Mains capacitors, for example so-called X2 capacitors, used in the capacitor power supply unit allow only low output currents of the power supply unit on account of their high reactance, with the result that, in addition to a controller or similar microprocessor, for example, additional small loads with a higher power requirement cannot be readily operated from the capacitor power supply unit. Such a small load may be an optical display, for example an optical display in the form of an LED. If the operating current of the LED is too low, the LED does not emit light or emits only very weak light, with the result that the optical display can scarcely be seen by the user, in particular in daylight.